I get this too with non iMessage texts. It's seems like it's been happening more lately. But what I did notice is that it mostly happens when I am at work. The coverage there like the Bermuda Triangle of cell service 🙄 It's awful at it's best no matter if I'm in the building or outside. We have wifi, but that doesn't help with regular text messages. I have a M-Cell at home and rarely get duplicate texts when I'm home. See if you have the issue in a particular location and maybe it's just the coverage where you are.